Resolving eustatic and tectonic influences in a sedimentary basin: upper Oligocene/lower Miocene of southern Patagonia, South America

PARRAS, A.; DIX, G.

Graz

Congreso; 2º International Congress on Stratigraphy (STRATI2015); 2015

University of Graz

Extensive high-latitude marine sedimentary rocks of late Oligocene-early Miocene age form the San Julián and overlying Monte León formations in the Austral Basin, southern Patagonia, South America. These deposits are important geological archives of tectonic, oceanographic, and climatic changes in response to development of the Drake Passage to the south, and sedimentary and diagenetic response of the Austral Basin to the Cenozoic compressional history of the Andean orogen to the west. The basin is an important paleogeographic location in which to develope regional climate and oceanographic frameworks spanning the Paleogene/Neogene boundary in the Southern Hemisphere. Integrated sedimentological and taphonomical studies of twelve sections in this part of Patagonia identify depositional conditions incorporating mixed (carbonate, siliciclastic) sediments of shallow marine platform to low-salinity marginal-marine environments. Paleoenvironmental analysis, facies stacking patterns, and Sr-isotope ages obtained from biogenic carbonate, establish an integrated stratigraphic framework to unravel relative sea-level changes in the local sedimentary basin to differentiate eustatic and tectonic signatures. Correlation with the global sea-level curve of Kominz et al. (2008) shows matches and mismatches. The lowest beds of the San Julián Formation register a deepening upward trend from marginal-marine to inner shelf that does not fit well with the global sea level curve. However, upper beds of inner shelf to subtidal environments correlate with global sea-level highstand ranging from 23.5 to 22.6 Ma. A glacioeustatic lowering in sea level in the earliest Miocene may correspond to a sequence boundary between the San Julián and Monte León formations that is well exposed in outcrops in the Puerto San Julián region. The significance of this correlation is threefold: (1) the lowermost Monte León Formation is of early Aquitanian age; (2) the Oligocene-Miocene boundary is positioned either within the uppermost San Julián Formation or lies between the two stratigraphic units; and (3) there is a mismatch between renewed transgression defined by the lowermost Monte León Formation and decrease in global sea level during the early Aquitanian. In the upper portion of this latter formation, well exposed at the mouth of the Santa Cruz River, and in Monte León National Park, inner-shelf to subtidal siltstone and fine tuffaceous sandstone with intercalated shell beds correlate with a global sea level stillstand of ~19 to 18 Ma. Sea-level fall in the mid Burdigalian (~18 Ma) may be responsible, in part, for a marked shift in coastline position towards the basin interior, resulting in accumulation of intertidal and marginal-marine facies in the uppermost part of this formation and lowermost strata of the overlying Santa Cruz Formation. Comparison of local sedimentation patterns with global sea-level change identifies the roles of local tectonic versus eustatic influences on deposition of sediments comprising the San Julián and Monte León formations. Tectonic controls in this foreland basin are related to development of the Patagonian Andes to the west. Although more integrated (e.g., bio, magneto- and chemostratigraphic) studies are necessary, the well-defined chronostratigraphic, facies, and sequence stratigraphic frameworks for this succession offer an unusually good opportunity for regional and supraregional correlations. This results in improved understanding of tectonic, paleoceanographic and paleoclimatic changes during the Paleogene/Neogene transition.